As far as why he sees me as primary, he says I'm the person he wants to spend the majority of his time with, the person he wants to come home to, the person he wants to build a life with over the long haul (and I feel the same way about him). But a little part of me wonders if there's more to it than that. Why seek out kinky/poly partners but then fall head over heels in love and pursue a primary relationship with an almost comically vanilla mono person? Is there something about my love for him and only him that's meeting an unacknowledged need or preference? And will his feelings for me change if I get emotionally involved with someone else?

Love is strange. I like the saying, "If you can explain the reasons 'why' then it's Like, not Love."

All of what you suggest is possible. But you can drive yourself crazy with worry by over-analyzing things.

Being poly, it's not "dangerous" for him to settle down with someone who doesn't meet 100% of his sexual/fetish needs, because he knows he can always get someone else to spank his tooshie. I personally don't operate that way, but it works for some people.

Very few of the people I've ever dated are the kinds of people I thought I was looking for, yet each of those relationships was satisfying in its own way. It's normal to have some insecurities and to question why someone is with you, but you can't let it rule your life.

__________________“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.”—bisexualbaker